Stable air tends toward stratiform clouds, poorer visibility, steady precipitation, and smooth air. Unstable air tends toward cumulus clouds, turbulence, and good visibility except in showers. Wind is air moving from higher pressure to lower pressure, and Coriolis force and friction only modify that flow.
What stable air produces
Compared with unstable air, stable air keeps vertical motion damped. Moisture that condenses spreads into layers instead of boiling upward. The clouds are stratiform. Precipitation, when it falls, tends to be steady rather than showery. The ride is smooth. Visibility is poorer, and the air is often hazy, because pollutants and moisture are not being mixed away by rising currents.
That is the full stable-air picture the questions use: stratiform clouds, steady precipitation, smooth air, and poorer visibility. Towering cumulus and good visibility belong to the unstable side. "No clouds under any condition" is not what stability means. Stable air can be full of cloud. It is the type of cloud, and the smoothness, that change. Stable air does not always have the best visibility. The visibility in stable air is the worse of the two patterns. If a choice says stable air is smooth but the visibility is excellent in every case, it has half of the sentence right and the half that matters wrong.
A warm front's long period of low cloud and steady rain fits this stable, layered pattern. Fog and a low ceiling are the surface version of the same layer, when the air is saturated. Those topics are on the ceiling and fog page. The point here is the stability label that goes with the layer.
What unstable air produces
Unstable air allows a parcel to keep rising once it starts. The clouds are cumuliform. The ride is turbulent. Visibility outside the showers is good, because the vertical motion mixes the air and carries haze up and out. In the showers themselves you can lose the horizon. The general statement still holds: unstable air tends to be turbulent but with good visibility, while stable air is smooth but often hazy.
Unstable air does not produce only stratus. Stratus is the stable-air cloud. If the sky is building cumulus and the aircraft is being bumped, you are in the unstable pattern, not the smooth one. Thunderstorms grow in unstable air, and the mature stage of a storm is the hazardous convective result. That stage, and the microburst that can come out of it, is on the thunderstorm page.
A fast-moving cold front is a common way the unstable pattern arrives: gusty wind, brief intense showers or thunderstorms, then clearing. After the front, visibility often improves. That improvement matches the "good visibility" side of unstable air once the shower has passed, and it matches the stable-versus-unstable contrast rather than a new rule.
What starts the wind
Wind is air moving from an area of higher pressure toward an area of lower pressure. The pressure difference is the cause. Once the air is moving, Coriolis deflection and friction modify the direction and the speed, especially near the surface where friction is greater. The question asks for the primary cause. The answer is the pressure difference. Naming Coriolis as the cause, instead of as a modifier, overshoots the explanation.
The drone's propellers do not cause the wind in the meteorological sense. A local puff from the rotors is not the wind the weather question is about. Temperature does not "always increase with altitude." In the usual lower atmosphere, temperature decreases as you climb. An increase with altitude is an inversion, which is associated with stable air near the surface, not with a definition of wind.
Density altitude uses temperature, pressure, and humidity to describe how thick the air is for the rotors. That is a performance number. It is not a second name for stability, and a high density altitude does not tell you whether the clouds will be stratus or cumulus.
A morning that is stable, and an afternoon that is not
The morning METAR shows haze, a low layered cloud, and light steady drizzle, with little bumpiness on a short hover. That is stable air: stratiform cloud, steady precipitation, smooth air, poor visibility. You watch the ceiling and the 3 statute mile visibility minimum before you commit to the job. By afternoon a front has passed, cumulus is building, the haze has broken, and the aircraft is being shoved around in the gusts. That is unstable air: turbulent, with good visibility away from the showers. The wind that arrived did so because pressure was unequal across the region. You do not explain the gusts by saying the propellers made them.
The trap
Assigning towering cumulus and clear air to the stable column, or stratus-only to the unstable column, swaps the two patterns. Giving stable air the best visibility ignores the haze. Explaining wind with propeller wash, or with temperature increasing with height, ignores the pressure difference. Stable is layered, smooth, and often hazy. Unstable is bumpy, cumuliform, and clearer outside showers. Wind starts as air moving from higher pressure to lower pressure, and Coriolis and friction modify it.
Try the stability items on the free practice test. Weather is taught in the course.